+===============================================================+ + i.MX6, i.MX7 U-Boot HABv4 Secure Boot guide for SPL targets + +===============================================================+ 1. HABv4 secure boot process ----------------------------- This document is an addendum of mx6_mx7_secure_boot.txt guide describing a step-by-step procedure on how to sign and securely boot an U-Boot image for SPL targets. Details about HAB can be found in the application note AN4581[1] and in the introduction_habv4.txt document. 1.1 Building a SPL target supporting secure boot ------------------------------------------------- The U-Boot provides Second Program Loader (SPL) support which generates two final images, SPL and U-Boot proper. The HABv4 can be used to authenticate both binaries. Out of reset the ROM code authenticates the SPL which is responsible for initializing essential features such as DDR, UART, PMIC and clock enablement. Once the DDR is available, the SPL code loads the U-Boot proper image to its specific execution address and call the HAB APIs to extend the root of trust. The U-Boot provides support to secure boot configuration and also provide access to the HAB APIs exposed by the ROM vector table, the support is enabled by selecting the CONFIG_IMX_HAB option. When built with this configuration the U-Boot correctly pads the final SPL image by aligning to the next 0xC00 address, so the CSF signature data generated by CST can be concatenated to the image. The U-Boot also append an Image Vector Table (IVT) in the final U-Boot proper binary (u-boot-ivt.img) so it can be used by HAB API in a post ROM stage. The diagram below illustrate a signed SPL image layout: ------- +-----------------------------+ <-- *start ^ | Image Vector Table | | +-----------------------------+ <-- *boot_data | | Boot Data | | +-----------------------------+ Signed | | Padding | Data | +-----------------------------+ <-- *entry | | | | | SPL | | | | | +-----------------------------+ v | Padding | ------- +-----------------------------+ <-- *csf | | | Command Sequence File (CSF) | | | +-----------------------------+ | Padding (optional) | +-----------------------------+ The diagram below illustrate a signed u-boot-ivt.img image layout: ------- +-----------------------------+ <-- *load_address ^ | | | | | | | u-boot.img | Signed | | | Data | | | | +-----------------------------+ | | Padding Next Boundary | | +-----------------------------+ <-- *ivt v | Image Vector Table | ------- +-----------------------------+ <-- *csf | | | Command Sequence File (CSF) | | | +-----------------------------+ | Padding (optional) | +-----------------------------+ 1.2 Enabling the secure boot support ------------------------------------- The first step is to generate an U-Boot image supporting the HAB features mentioned above, this can be achieved by adding CONFIG_IMX_HAB to the build configuration: - Defconfig: CONFIG_IMX_HAB=y - Kconfig: ARM architecture -> Support i.MX HAB features 1.3 Creating the CSF description file -------------------------------------- The CSF contains all the commands that the HAB executes during the secure boot. These commands instruct the HAB code on which memory areas of the image to authenticate, which keys to install, use and etc. CSF examples are available under doc/imx/habv4/csf_examples/ directory. Build logs containing the "Authenticate Data" parameters are available after the U-Boot build, the example below is a log for mx6sabresd_defconfig target: - SPL build log: $ cat SPL.log Image Type: Freescale IMX Boot Image Image Ver: 2 (i.MX53/6/7 compatible) Mode: DCD Data Size: 69632 Bytes = 68.00 KiB = 0.07 MiB Load Address: 00907420 Entry Point: 00908000 HAB Blocks: 0x00907400 0x00000000 0x0000ec00 - u-boot-ivt.img build log: $ cat u-boot-ivt.img.log Image Name: U-Boot 2019.01-00003-g78ee492eb3 Created: Mon Jan 14 17:58:10 2019 Image Type: ARM U-Boot Firmware with HABv4 IVT (uncompressed) Data Size: 458688 Bytes = 447.94 KiB = 0.44 MiB Load Address: 17800000 Entry Point: 00000000 HAB Blocks: 0x177fffc0 0x0000 0x0006e020 As explained in section above the SPL is first authenticated by the ROM code and the root of trust is extended to the U-Boot image, hence two CSF files are necessary to completely sign a bootloader image. In "Authenticate Data" CSF command users can copy and past the output addresses, the csf_uboot.txt can be used as example: - In csf_SPL.txt: Block = 0x00907400 0x00000000 0x0000ec00 "SPL" - In csf_uboot-ivt.txt: Block = 0x177fffc0 0x0000 0x0006e020 "u-boot-ivt.img" 1.4 Signing the images ----------------------- The CST tool is used for singing the U-Boot binary and generating a CSF binary, users should input the CSF description file created in the step above and receive a CSF binary, which contains the CSF commands, SRK table, signatures and certificates. - Create SPL CSF binary file: $ ./cst -i csf_SPL.txt -o csf_SPL.bin - Append CSF signature to the end of SPL image: $ cat SPL csf_SPL.bin > SPL-signed - Create U-Boot proper CSF binary file: $ ./cst -i csf_uboot-ivt.txt -o csf_uboot-ivt.bin - Append CSF signature to the end of U-Boot proper image: $ cat u-boot-ivt.img csf_uboot-ivt.bin > u-boot-signed.img The bootloader is signed and can be flashed into the boot media. 1.5 Closing the device ----------------------- The procedure for closing the device is similar as in Non-SPL targets, for a complete procedure please refer to section "1.5 Programming SRK Hash" in mx6_mx7_secure_boot.txt document available under doc/imx/habv4/guides/ directory. References: [1] AN4581: "Secure Boot on i.MX 50, i.MX 53, i.MX 6 and i.MX 7 Series using HABv4" - Rev 2.